As a measure for reducing electromagnetic interference (EMI) inherent in this type of image-reading device, the use of spread-spectrum clock generation has been proposed for various electronic devices incorporating such an image-reading device in order to modulate the clock frequency over a constant modulation period. However, if the irradiation time of the light source in the image-reading device and the reading time of the reading unit in the same are set based on prescribed numbers of clock cycles of a spread-spectrum clock, the irradiation time and reading time will also vary in response to changes in the frequency of the spread-spectrum clock. However, if the irradiation time or reading time is fluctuating, the reading unit cannot accurately read the image on the original which moves at a constant speed relative to the reading unit. Japanese Patent Application Publication No. 2006-332924 has proposed a method to set the irradiation time to an integral multiple of the modulation period for the spread-spectrum clock, thereby setting the irradiation time to a constant value that is not influenced by changes in the frequency of the spread-spectrum clock. United States Patent Application Publication No. 2004/013321 A1 has proposed a method to set the reading time to an integral multiple of the modulation period for the spread-spectrum clock, thereby setting the reading time to a constant value that is not influenced by changes in the frequency of the spread-spectrum clock.